Benefit of high resolution COSMO reanalysis: The diurnal cycle of column-integrated water vapor over Germany

Abstract

The ability of the two novel high resolution reanalyses, COSMO-REA with grid resolution of 2 (REA2) and 6 (REA6) km, to represent integrated water vapor (IWV) is investigated relative to the much coarser, i.e. approx. 80 km resolution, global reanalysis ERA-Interim. The comparison with reference data from 133 Global Positioning System (GPS) stations distributed over Germany reveals the clear superiority of the COSMO-REA products for providing instantaneous IWV with a median standard deviation of 1.6 kg m−2 compared to 2.4 kg m−2 for ERA-Interim. The improvement is still visible on the daily and even monthly scale and is attributed to a better representation of surface heterogeneity and boundary-layer processes on small scales. The IWV diurnal cycle being mostly influenced by boundary layer processes is analyzed in detail to assess the realism of COSMO-REA. On average both COSMO-REA show similar characteristics of the IWV diurnal cycle as GPS measurements, however, with reduced amplitude. Reanalyses and GPS show the strongest amplitude of the diurnal cycle in spring followed by summer. The lower amplitude of COSMO-REA could be traced down to the missing seasonal dependence of vegetation parameters resulting in too low transpiration during the vegetation phase. Still COSMO-REA is able to reproduce the dependency of the amplitude in respect to orography and the synoptic situation.

Keywords

• high resolution reanalysis
• water vapor
• diurnal cycle
• GPS